The increasingly widespread use of complex samples and advanced spectroscopic detectors such as mass spectrometers has dramatically broadened the utility and information yield of analytical chromatographic separations. Earlier detection techniques such as refractometry or fixed wavelength ultraviolet absorbance detection might imply the presence of a suitable analyte within the detection volume, and with the use of known calibrators, might imply the concentration of the analyte. However, the identity of the analyte, at best, was inferred by comparison with the chromatographic retention time or retention volume of a known standard. Identification of the analyte was effectively not a property or a capability of such detection subsystems.
Such detectors were also susceptible to significant quantitation errors in the presence of overlapping chromatographic bands or zones, limiting their utility in the analysis of highly complex mixtures as are commonly encountered in biological and environmental applications. Historically, the application of mass spectrometry to chromatographic detection facilitated analyte detection within vastly more complex mixtures, by permitting high resolution separation in the mass-to-charge domain to augment chromatographic separation in the fluid volume domain.
Depending upon the mode of mass spectrometric analysis being performed, and the nature of the analyte(s), putative compound identification might be performed in line during the separation, substantially without reliance upon the chromatographic retention time or retention volume of a standard. There exist, however, important classes of compounds for which mass spectrometry alone is not capable of rendering a full and complete identification. Commonly, examples are found where isomerization is present, where the isomers share or exhibit the same chemical formula and parent mass to charge ratio, but are assembled in arrangements which can be either structurally or spatially distinct. Knowledge of the chemical formula, while useful, is incomplete if the configuration or arrangement of the molecule is questionable or fully unknown. Isomers can have distinctly (in some cases, radically) different behaviors within biological systems, if supplied as pharmaceutical compounds, or if rendered as degradation products.